U.S. patent application number 10/477753 was filed with the patent office on 2004-12-09 for optical wavelength conversion method, optical wavelength conversion system, program and medium, and laser oscillation system.
Invention is credited to Kato, Hirokazu, Masada, Genta, Sekine, Ichiro, Shiraishi, Hiroyuki, Watanabe, Noriko.
Application Number | 20040246565 10/477753 |
Document ID | / |
Family ID | 27531906 |
Filed Date | 2004-12-09 |
United States Patent
Application |
20040246565 |
Kind Code |
A1 |
Sekine, Ichiro ; et
al. |
December 9, 2004 |
Optical wavelength conversion method, optical wavelength conversion
system, program and medium, and laser oscillation system
Abstract
In an optical wavelength converting method in which light from a
laser oscillator that oscillates coherent light of an inherent
wavelength .lambda. is employed as incident light, and is made to
input to a nonlinear optical crystal, and light having a wavelength
of 1/2 .lambda. is radiated, the wavelength of the incident light
is 1000 nm or less, and the peak power density of the incident
light is 0.1-10 fold greater than the peak power density that
provides the maximum conversion efficiency. The nonlinear optical
crystal is heated to and maintained at 200-600.degree. C. An
optical wavelength converting method, and a program therefor are
provided that achieves high conversion efficiency stably using an
nonlinear optical crystal, enabling manufacture of an all solid
state ultraviolet laser oscillator that is durable with respect to
practical application. Prescribed fundamental waves are input to a
first crystal and a second crystal sequentially. The first crystal
has a higher bulk damage threshold value with respect to the
fundamental waves than that of the second crystal. The second
crystal has a higher effective nonlinear constant with respect to
the fundamental waves than that of the first crystal. An optical
wavelength converting method, optical wavelength converting system
and a laser oscillating system are provided that enable high power
second harmonics to be obtained with good efficiency.
Inventors: |
Sekine, Ichiro;
(Okegawa-shi, JP) ; Shiraishi, Hiroyuki;
(Saitama-shi, JP) ; Kato, Hirokazu; (Naka-gun,
JP) ; Masada, Genta; (Naka-gun, JP) ;
Watanabe, Noriko; (Shiroi-shi, JP) |
Correspondence
Address: |
OBLON, SPIVAK, MCCLELLAND, MAIER & NEUSTADT, P.C.
1940 DUKE STREET
ALEXANDRIA
VA
22314
US
|
Family ID: |
27531906 |
Appl. No.: |
10/477753 |
Filed: |
June 30, 2004 |
PCT Filed: |
May 23, 2002 |
PCT NO: |
PCT/JP02/05005 |
Current U.S.
Class: |
359/326 |
Current CPC
Class: |
G02F 1/3507 20210101;
G02F 1/37 20130101; G02F 1/3534 20130101; G02F 1/3525 20130101 |
Class at
Publication: |
359/326 |
International
Class: |
G02F 001/35 |
Foreign Application Data
Date |
Code |
Application Number |
May 25, 2001 |
JP |
2001-157289 |
Claims
1. An optical wavelength converting method in which light of a
prescribed repetition frequency from a laser oscillator that
oscillates coherent light of an inherent wavelength .lambda. is
employed as incident light, and is made to input to a nonlinear
optical crystal having a prescribed crystal length, and light
having a wavelength of {fraction (1/2)} .lambda. is radiated,
wherein the wavelength of the incident light is 1000 nm or less,
and the peak power density of the incident light is 0.1-10 fold
greater than the peak power density that provides the maximum
conversion efficiency.
2. An optical wavelength converting method according to claim 1,
wherein the nonlinear optical crystal is a lithium tetraborate
(Li.sub.2B.sub.4O.sub.7) single crystal.
3. An optical wavelength converting method in which light of a
prescribed repetition frequency from a laser oscillator that
oscillates coherent light of an inherent wavelength .lambda. is
employed as incident light and is made to input to a lithium
tetraborate (Li.sub.2B.sub.4O.sub.7) single crystal having a
prescribed crystal length, and light having a wavelength of
{fraction (1/2)} .lambda. is radiated, wherein the wavelength of
the incident light is 1000 nm or less, and the peak power density
of the incident light is 0.1-10 fold greater than the optimal peak
power density Pc obtained by the following formula (1).
Pc=.alpha..multidot.Rep.sup..beta. (1) (Where: Rep=repetition
frequency, and .alpha. and .beta. are constants.)
4. An optical wavelength converting method according to claim 1,
wherein the incident light has a beam spreading of 10 m rad or
less, a time pulse width of 100 n sec or less, and a peak power
density of 1 MW/cm.sup.2 or greater.
5. An optical wavelength converting system provided with a laser
oscillator for oscillating coherent light having an inherent
wavelength A, and a nonlinear optical crystal of a prescribed
crystal length in which light of a prescribed repetition frequency
from the laser oscillator is employed as incident light, and light
having a wavelength of {fraction (1/2)} .lambda. is radiated,
wherein the wavelength of the incident light is 1000 nm or less,
and the peak power density of the incident light is 0.1-10 fold
greater than the peak power density that provides the maximum
conversion efficiency.
6. An optical wavelength converting system that is provided with a
laser oscillator for oscillating coherent light of an inherent
wavelength A, and a lithium tetraborate (Li.sub.2B.sub.4O.sub.7)
single crystal of a prescribed crystal length in which light of a
prescribed repetition frequency from the laser oscillator is
employed as incident light, and light having a wavelength of
{fraction (1/2)} .lambda. is radiated, wherein the wavelength of
the incident light is 1000 nm or less, and the peak power density
of the incident light is 0.1-10 fold greater than the optimal peak
power density Pc obtained from the following Formula (1).
Pc=.alpha..multidot.Rep.sup..beta. (1) (Where: Rep=repetition
frequency, and .alpha. and .beta. are constants.)
7. A program for activating a computer comprising: an input section
for receiving a data group comprising the peak power density and
conversion efficiency of incident light when radiated light of
wavelength of {fraction (1/2)} .lambda. is obtained by causing
incident light of a prescribed repetition frequency and wavelength
.lambda. to input to a nonlinear optical crystal; a memory section
in which a plurality of the data groups is stored; a calculating
section for calculating the peak power density at which the maximum
conversion efficiency is obtained using the plurality of data
groups stored in the memory section; and an output section for
outputting the peak power density at which the maximum conversion
efficiency can be obtained that is calculated by the calculating
section.
8. A program for activating a computer comprising: an input section
for receiving preset values comprising the repetition frequency Rep
and constants .alpha. and P when obtaining radiated light of
wavelength {fraction (1/2)} .lambda. by causing light of wavelength
.lambda. to input to a nonlinear optical crystal; a calculating
section for calculating the optimal peak power density based on the
following Formula (I) using the preset values input into the input
section; and an output section for outputting the optimal peak
power density obtained by the calculating section.
Pc=.alpha..multidot.Rep.sup..beta. (1) (Where: Rep=repetition
frequency, and .alpha. and .beta. are constants.)
9. A computer readable medium for holding a program which activates
a computer comprising: an input section for receiving a data group
comprising the peak power density and conversion efficiency of
incident light when radiated light of wavelength of {fraction
(1/2)} .lambda. is obtained by causing incident light of a
prescribed repetition frequency and wavelength .lambda. to input to
a nonlinear optical crystal; a memory section for storing a
plurality of the data groups; a calculating section for calculating
the peak power density at which the maximum conversion efficiency
is obtained using the plurality of data groups stored in the memory
section; and an output section for outputting the peak power
density at which the maximum conversion efficiency can be obtained
that is calculated by the calculating section.
10. A computer readable medium for holding a program which
activates a computer comprising: an input section for receiving
preset values comprising the repetition frequency Rep and constants
.alpha. and .beta. when radiated light of wavelength of {fraction
(1/2)} .lambda. is obtained by causing incident light of wavelength
.lambda. to input to a nonlinear optical crystal; a calculating
section for calculating the optimal peak power density based on the
following Formula (1) using the preset values input into the input
section; and an output section for outputting the optimal peak
power density obtained by the calculating section.
Pc=.alpha..multidot.Rep.sup..beta. (1) (Where: Rep=repetition
frequency, and .alpha. and .beta. are constants.)
11. An optical wavelength converting method which employs light
from a laser oscillator that oscillates coherent light of inherent
wavelength .lambda. as incident light, inputs the light to a
nonlinear optical crystal and radiates out light of wavelength
{fraction (1/2)} .lambda., wherein the nonlinear optical crystal is
heated to and maintained at 200-600.degree. C.
12. An optical wavelength converting method according to claim 11,
wherein the wavelength of the incident light is 1000 nm or
less.
13. An optical wavelength converting method according to claim 11,
wherein the incident light has beam spreading of 10 m rad or less,
a time pulse width of 100 n sec or less and a peak power density of
1 MW/cm.sup.2 or more.
14. An optical wavelength converting system comprising: a laser
oscillator that oscillates coherent light of inherent wavelength
.lambda.; a nonlinear optical crystal in which light from the laser
oscillator is employed as incident light, and light of wavelength
{fraction (1/2)} .lambda. is radiated; and a heating section that
heats and maintains the nonlinear optical crystal at
200-600.degree. C.
15. An optical wavelength converting method in which light from a
laser oscillator, that oscillates coherent light of inherent
wavelength .lambda. as incident light, is input to a lithium
tetraborate (Li.sub.2B.sub.4O.sub.7) single crystal, and light of
wavelength {fraction (1/2)} .lambda. is radiated, wherein the
lithium tetraborate (Li.sub.2B.sub.4O.sub.7) single crystal is
heated to and maintained at 50-600.degree. C.
16. An optical wavelength converting method according to claim 15,
wherein the wavelength of the incident light is 1000 nm or
less.
17. An optical wavelength converting method according to claim 15,
wherein the incident light has beam spreading of 10 m rad or less,
a time pulse width of 100 n sec or less, and a peak power density
of 1 MW/cm.sup.2 or more.
18. An optical wavelength converting system comprising: a laser
oscillator that oscillates coherent light of an inherent wavelength
.lambda.; a lithium tetraborate (Li.sub.2B.sub.4O.sub.7) single
crystal that employs light from the laser oscillator as incident
light and radiates light of wavelength {fraction (1/2)} .lambda.;
and a heating section that heats and maintains the lithium
tetraborate single crystal at 50-600.degree. C.
19. An optical wavelength converting method in which fundamental
waves of a prescribed wavelength and time pulse width are input to
a first nonlinear optical crystal and a second nonlinear optical
crystal sequentially, and the second harmonic of the fundamental
waves is generated, wherein the bulk damage threshold of the first
nonlinear optical crystal with respect to the fundamental wave is
larger than that of the second nonlinear optical crystal, and the
effective nonlinear constant of the second nonlinear optical
crystal with respect to the second harmonic wave generation of the
fundamental waves is larger than that of the first nonlinear
optical crystal.
20. An optical wavelength converting method in which a first
fundamental wave of a prescribed wavelength and time pulse width
and a second fundamental wave of a prescribed wavelength and time
pulse width are input to a first nonlinear optical crystal and a
second nonlinear optical crystal sequentially, and the sum
frequency wave of the first fundamental wave and the second
fundamental wave are generated, wherein the bulk damage threshold
of the first nonlinear optical crystal with respect to the first
fundamental wave is larger than that of the second nonlinear
optical crystal, and the effective nonlinear constant of the second
nonlinear optical crystal with respect to the sum frequency wave
generation from the first fundamental wave and the second
fundamental wave is larger than that of the first nonlinear optical
crystal.
21. An optical wavelength converting method according to claim 19,
wherein the first nonlinear optical crystal is lithium tetraborate
(Li.sub.2B.sub.4O.sub.7) single crystal.
22. An optical wavelength converting method according to claim 21,
wherein the second nonlinear optical crystal is LiB.sub.3O.sub.5,
CsLiB.sub.6O.sub.10, KTiOPO.sub.4, or .beta.-BaB.sub.2O.sub.4.
23. An optical wavelength converting system comprising: a first
nonlinear optical crystal to which a fundamental wave of a
prescribed wavelength and time pulse width is input and a second
harmonic wave is generated; and a second nonlinear optical crystal
to which radiated light from the first nonlinear optical crystal is
input and a second harmonic wave of the fundamental wave is
generated, wherein the bulk damage threshold of the first nonlinear
optical crystal with respect to the fundamental wave is larger than
that of the second nonlinear optical crystal, and the effective
nonlinear constant of the second nonlinear optical crystal with
respect to the second harmonic wave generation of the fundamental
wave is larger than that of the first nonlinear optical
crystal.
24. An optical wavelength converting system comprising: a first
nonlinear optical crystal to which a first fundamental wave of a
prescribed wavelength and time pulse width and a second fundamental
wave of a prescribed wavelength and time pulse width are input, and
the sum frequency wave of the first fundamental wave and the second
fundamental wave is generated; and a second nonlinear optical
crystal to which radiated light from the first nonlinear optical
crystal is input and the sum frequency wave is generated, wherein
the bulk damage threshold of the first nonlinear optical crystal
with respect to the first fundamental wave is larger than that of
the second nonlinear optical crystal, and the effective nonlinear
constant of the second nonlinear optical crystal with respect to
the sum frequency wave generation from the first fundamental wave
and the second fundamental wave is larger than that of the first
nonlinear optical crystal.
25. A laser oscillating system comprising: a fundamental wave
oscillator for oscillating a fundamental wave of a prescribed
wavelength and time pulse width; and an optical wavelength
converting system to which the fundamental wave from the
fundamental wave oscillator is input and a second harmonic wave is
generated, wherein the optical wavelength converting system is the
optical wavelength converting system according to claim 23.
26. A laser oscillating system comprising: a fundamental wave
oscillator for oscillating a first fundamental wave of a prescribed
wavelength and time pulse width and a second fundamental wave of a
prescribed wavelength and time pulse width; and an optical
wavelength converting system to which the first fundamental wave
and the second fundamental wave from the fundamental wave
oscillator are input and a sum frequency wave is generated, wherein
the optical wavelength converting system is the optical wavelength
converting system according to claim 24.
27. An optical wavelength converting method according to claim 2,
wherein the incident light has a beam spreading of 10 m rad or
less, a time pulse width of 100 n sec or less, and a peak power
density of 1 MW/cm.sup.2 or greater.
28. An optical wavelength converting method according to claim 3,
wherein the incident light has a beam spreading of 10 m rad or
less, a time pulse width of 100 n sec or less, and a peak power
density of 1 MW/cm.sup.2 or greater.
29. An optical wavelength converting method according to claim 12,
wherein the incident light has beam spreading of 10 m rad or less,
a time pulse width of 100 n sec or less and a peak power density of
1 MW/cm.sup.2 or more.
30. An optical wavelength converting method according to claim 16,
wherein the incident light has beam spreading of 10 m rad or less,
a time pulse width of 100 n sec or less, and a peak power density
of 1 MW/cm.sup.2 or more.
31. An optical wavelength converting method according to claim 20,
wherein the first nonlinear optical crystal is lithium tetraborate
(Li.sub.2B.sub.4O.sub.7) single crystal.
Description
TECHNICAL FIELD
[0001] The present invention relates to an optical wavelength
converting method, and an optical wavelength converting system,
program, and medium, that are employed in a laser oscillator. More
specifically, the present invention relates to an optical
wavelength converting method, and to an optical wavelength
converting system, program and data medium, wherein coherent light
is input to a nonlinear optical crystal, and in particular on a
lithium tetraborate (Li.sub.2B.sub.4O.sub.7, referred to as "LB4"
hereinafter) single crystal, that is employed as a second harmonic
wave generation element, and then radiates this light after
converting it to light having one-half the original wavelength. In
addition, the present invention relates to an optical wavelength
converting method, an optical wavelength converting system, and a
laser oscillating system that are capable of obtaining with good
efficiency second harmonic, third harmonic and other such high
power sum frequency waves.
BACKGROUND ART
[0002] When employed as the light source for recording data into or
reading data out from a recording medium, short-wavelength laser
light has the advantage of enabling an increased recording density.
In addition, short-wavelength laser light is also advantageous when
employed in material processing applications, as its heat effects
are small and it makes precision processing possible.
Short-wavelength laser light is also being used such as a light
source in the medical field, and a lithography light source for a
very large-scale integrated circuit.
[0003] Thus, short-wavelength laser light is desired in many
diverse fields. Accordingly, there has been a demand for a small,
lightweight, long-lasting light source that stably radiates
short-wavelength laser light.
[0004] However, a suitable light source that radiates light having
a wavelength of 500 nm or less has not been conventionally
available. For example, while semiconductor lasers are known that
can radiate laser light having wavelengths of up to 400 nm, these
devices have been problematic because of their extremely low
output.
[0005] Excimer lasers are available as examples of short-wavelength
large-output lasers. These lasers were first realized in 1970 by
Basov et al in the former Soviet Union using a method of exciting
liquid xenon (Xe) with an electronic beam. In 1976, these lasers
were successfully oscillated using electric discharge pumping. In
excimer lasers of this type, i.e., employing electric discharge
pumping, ultraviolet light is generated by compounds such as ArF
(193 nm), KrF (248 nm), or XeCl (308 nm) in an ultraviolet pulse
repetition oscillating laser, amplified using an optical resonator,
and then output as laser light. Application of excimer lasers has
been much anticipated in fields such as polymer ablation, surface
reforming, marking, thin film formation, medical product
manufacturing, and isotope separation. However, when pulse lasers
which repeatedly generate several hundred pulses per second are
used as excimer lasers, they can only generate a 10.sup.-9 second
pulse light every 10.sup.-2 seconds. That is, the duration during
which the laser is being generated is extremely short compared to
the interval, so that application of excimer lasers in a deposition
process or the processing steps employed in the aforementioned
fields is problematic. Furthermore, excimer lasers are also
problematic with respect to the short lifespan of the gas medium,
difficulty in reducing the size of the laser device, poor
maintenance, high operational costs, employment of toxic gases,
etc. Thus, the practical utilization of semiconductor lasers, etc.
that can generate light in the ultraviolet region at room
temperature, stably and over a long period of time, has yet to be
realized.
[0006] There has therefore been increased research activity in
recent years in the area of nonlinear optical elements such as
second harmonic-wave generating (SHG) elements. SHG elements
generate light having one-half the wavelength of the incident light
so that, for example, light in the ultraviolet region can be
generated using laser light in the infrared region. Thus, the
industrial value of this technology in various fields of
application is extremely large.
[0007] Conventionally known crystals employed as wavelength
converting elements like SHG elements include KTP (KTiOPO.sub.4)
disclosed in Japanese Unexamined Patent Application, First
Publication No. Hei 3-65597, and BBO (.beta.-BaB.sub.2O.sub.4),
CLBO (CsLiB.sub.61O.sub.10), LBO (LiB.sub.3O.sub.5), and KDP
(KH.sub.2PO.sub.4), etc. disclosed in Japanese Unexamined Patent
Application, First Publication No. Sho 63-279231.
[0008] However, in the case of a wavelength converting element
employing KTP, not only is it difficult to increase the size of the
crystal, but the refractive index varies inside the crystal.
Accordingly, even in the case of KTP elements that are cut from a
single crystal, the refractive indices will differ from one
another. As a result, the phase matching angles differ, making it
difficult to realize a wavelength converting element that is highly
precise. Further, since pores are readily generated in a KTP type
crystal, it is difficult to supply a large amount of high-quality
KTP crystals.
[0009] In addition, while converting elements employing BBO or CLBO
have high conversion efficiency, they are problematic with respect
to resistance to moisture and laser damage, and output
destabilization due to two photon absorption.
[0010] In converting elements employing LBO, the shortest SHG
wavelength (second harmonic wave) is 277 nm, so that the wavelength
conversion range is narrow. For this reason, these devices cannot
generate the fourth harmonic wave (266 nm) of an Nd:YAG laser.
Further, another disadvantage is that a large crystal is not
possible.
[0011] In converting elements employing KDP, phase mismatching
arises due to the effects of heat absorbed at a high repetition
rate. Accordingly, these elements cannot be used unless a low
repetition rate of 100 Hz or less is employed. In addition, at a
high repetition rate, the threshold for damage is extremely low.
Accordingly, it is difficult to employ this device in laser
oscillators used in manufacturing or industrial applications that
are employed at repetition rates exceeding 1 kHz.
[0012] The present applicant therefore previously proposed a
wavelength converting method employing an LB4
(Li.sub.2B.sub.4O.sub.7) single crystal as a converting element
(Japanese Patent Application No. Hei 8-250523).
[0013] This LB4 single crystal is highly transmissive with respect
to a wide range of wavelengths and incurs little damage from the
laser light. Further, a large crystal with excellent quality can be
manufactured easily. In addition, this LB4 single crystal is
superior with respect to workability, low deliquescence, and
excellent ease of handling. In addition, this crystal has a long
lifespan.
[0014] Accordingly, a small, lightweight, inexpensive optical
converting element can be realized using LB4 that can be operated
stably over a long period of time, has a long lifespan, and
excellent workability.
[0015] The conversion efficiency of a wavelength converting element
is determined mainly by the inherent physical properties of the
crystal, such as its nonlinear optical constant and the tolerance
zone for the phase matching angle. An LB4 single crystal has the
disadvantage of low conversion efficiency when compared to BBO and
CLBO. For this reason, it was felt that an LB4 single crystal with
its low conversion efficiency was not suitable for use as a
wavelength converting element for radiating light in the
ultraviolet region.
[0016] In order to improve the low conversion efficiency and obtain
radiated light of a high average output, a variety of technical
methods can be employed. Conventionally employed methods include,
for example, increasing the peak power density of the incident
light by using a lens to converge the incident light; increasing
the crystal length; using a plurality of wavelength converting
crystals; and employing as the light source a laser oscillator that
has high quality beam characteristics, i.e., little beam spreading
at high outputs.
[0017] However, improving the conversion efficiency using these
types of technical methods has had the following limitations.
[0018] First, in the method for increasing the peak power density
of the incident light by converging the incident light with a lens,
the peak power density cannot be increased limitlessly; rather,
consideration must be given to laser damage from the incident
light.
[0019] In other words, an antireflection film to reduce reflection
is typically coated onto the end face of the crystal element in the
wavelength converting element. However, in general, this
antireflection film's resistance to damage by the laser is not all
that sufficient, so that damage can be incurred if the peak power
density of the incident light is high. In addition, when the light
is input at a high peak power density, it is possible for the
crystal element itself to suffer dielectric breakdown. Accordingly,
the wavelength converting element's laser damage threshold,
including the characteristics of the antireflection film, must be
taken into consideration, and appropriate limits then applied to
the peak power density of the incident light.
[0020] In addition, even in the case where high conversion
efficiency is obtained by increasing the peak power density of the
incident light, nonlinear optical crystals have the unique problem
of two photon absorption. This is a phenomenon whereby, as a result
of two photon absorption by the crystal itself, a donut-shaped hole
opens up in the center of the radiated light beam pattern, leading
to extremely unstable output. Two photon absorption can strengthen
in proportion to the square of the beam intensity of the radiated
light. Thus, heating within the crystal from absorption can have a
large effect, particularly at the high intensity beam center,
causing the refractive index to vary and disrupting phase
matching.
[0021] Note that for the purpose of protecting nonlinear crystals
from moisture, or to perform phase matching using temperature, it
has been the conventional practice to heat and maintain nonlinear
optical crystals at 40-200.degree. C.
[0022] When a lens is used to converge incident light, spreading of
the incident beam increases. As a result, the tolerance zone for
the phase matching angle is exceeded, and conversion efficiency
decreases.
[0023] In the case of the method in which crystal length is
increased, the tolerance zone for the phase matching angle narrows
and absorption by the crystal increases when the crystal is made
longer. Once a specific length has been exceeded, there is a
tendency for the conversion efficiency to gradually become
saturated. In addition, strain arises in the beam pattern from
walk-off when the crystal becomes longer. Thus, this crystal
lengthening approach, as well, cannot be deemed entirely
effective.
[0024] In the method employing a plurality of individual wavelength
converting crystals, a beam passes through a crystal without
undergoing wavelength conversion is reused by being made to input
to the next crystal. In this method, not only does the conversion
efficiency increase, but an increased output may be expected from
the effects of interference between wavelength converted light
generated by the plurality of individual crystals. However, when
there is broad spreading of the incident light beam, or when the
beam diameter is small in this method, it is not possible to obtain
a sufficient interference effect.
[0025] In the case of the method in which a laser oscillator having
high quality beam characteristics is employed as the light source,
use of a beam that experiences little spreading at high power is
certainly ideal from the perspective of increasing conversion
efficiency. However, it is difficult to make this type of
oscillator at low cost.
[0026] Further, as an additional problem, as explained above, while
use of various nonlinear optical crystals as converting elements is
known, a method has not yet been achieved that enables second
harmonic waves and other such high power sum frequency waves to be
obtained with good efficiency.
[0027] In other words, in order to obtain high power sum frequency
waves like second harmonic waves, it is first necessary to employ a
converting element that can achieve a high conversion efficiency.
Secondly, in order to enable conversion of high power incident
light, it is necessary to use a converting element that possesses
high resistance to damage from the incident light.
[0028] However, it is generally the case that nonlinear optical
crystals that have high conversion efficiency have poor resistance
to damage, while nonlinear optical crystals that are highly
resistant to damage have poor conversion efficiency. Thus, a
nonlinear crystal equipped with both sufficient conversion
efficiency and resistance to damage has not been available.
[0029] The present invention was conceived in view of the
above-described problems and is directed to the provision of an
optical wavelength converting method, and to an optical wavelength
converting system, program and medium, which enable production of
an all solid state ultraviolet laser oscillator that stably
achieves a high conversion efficiency using a nonlinear optical
crystal, lithium tetraborate single crystal LB4 for example, and is
durable with respect to practical applications (first problem).
[0030] The present invention is further directed to the provision
of an optical wavelength converting method, an optical wavelength
converting system, and a laser oscillating system that compensate
for the restrictive conditions of the nonlinear optical crystals
that can be employed and are capable of obtaining high power sum
frequency waves such as second harmonic waves with good efficiency
(second problem).
DISCLOSURE OF INVENTION
[0031] In order to resolve the above-described first problem, the
present invention provides an optical wavelength converting method
in which light of a prescribed repetition frequency from a laser
oscillator that oscillates coherent light of an inherent wavelength
.lambda. is employed as incident light, and is made to input to a
nonlinear optical crystal having a prescribed crystal length, and
light having a wavelength of {fraction (1/2)} .lambda. is radiated,
wherein the wavelength of the incident light is 1000 nm or less,
and the peak power density of the incident light is 0.1-10 fold
greater than the peak power density that provides the maximum
conversion efficiency.
[0032] Further, the present invention provides an optical
wavelength converting method in which light of a prescribed
repetition frequency from a laser oscillator that oscillates
coherent light of an inherent wavelength .lambda. is employed as
incident light and is made to input to a lithium tetraborate
(Li.sub.2B.sub.4O.sub.7) single crystal having a prescribed crystal
length, and light having a wavelength of {fraction (1/2)} .lambda.
is radiated; wherein the wavelength of the incident light is 1000
nm or less, and the peak power density of the incident light is
0.1-10 fold greater than the optimal peak power density Pc obtained
by the following formula (1)
Pc=.alpha..multidot.Rep.sup..beta. (1)
[0033] (Where: Rep=repetition frequency, and .alpha. and .beta. are
constants.)
[0034] The present invention further provides an optical wavelength
converting system comprising: a laser oscillator for oscillating
coherent light having an inherent wavelength .lambda., and a
nonlinear optical crystal of a prescribed crystal length in which
light of a prescribed repetition frequency from the laser
oscillator is employed as incident light, and light having a
wavelength of {fraction (1/2)} .lambda. is radiated, wherein the
wavelength of the incident light is 1000 nm or less, and the peak
power density of the incident light is 0.1-10 fold greater than the
peak power density that provides the maximum conversion
efficiency.
[0035] In addition, the present invention provides an optical
wavelength converting system that is provided with a laser
oscillator for oscillating coherent light of an inherent wavelength
.lambda., and a lithium tetraborate (Li.sub.2B.sub.4O.sub.7) single
crystal of a prescribed crystal length in which light of a
prescribed repetition frequency from the laser oscillator is
employed as incident light, and light having a wavelength of
{fraction (1/2)} .lambda. is radiated; wherein the wavelength of
the incident light is 1000 nm or less, and the peak power density
of the incident light is 0.1.about.10 fold greater than the optimal
peak power density Pc obtained by the following Formula (1).
Pc=.alpha..multidot.Rep.sup..beta. (1)
[0036] (Where: Rep=repetition frequency, and .alpha. and .beta. are
constants.)
[0037] The peak power density of the incident light in the each of
the respective inventions as described above is 0.1-10 fold,
preferably 0.1-5 fold, and even more preferably 0.5-2 fold greater
than the peak power density that provides the maximum conversion
efficiency.
[0038] The wavelength of the incident light in the respective
inventions as described above is 1000 nm or less. However, a
preferable range is 400-800 nm, a more preferable range is 400-600
nm, an even more preferable range is 400-550 nm, and the most
preferable range is 480-540 nm.
[0039] It is desirable that beam spreading of the incident light in
the respective inventions as described above is 10 m rad or less,
and more preferably is in the range of 0.3-4 m rad.
[0040] It is preferable that the time pulse width is 100 n sec or
less, with 1.times.10.sup.-3-80 n sec range being more
preferable.
[0041] It is desirable that the peak power density be 1 MW/cm.sup.2
or greater.
[0042] In addition, the present invention provides a program for
activating a computer comprising: an input section for receiving a
data group comprising the peak power density and conversion
efficiency of incident light when radiated light of wavelength of
{fraction (1/2)} .lambda. is obtained by causing incident light of
a prescribed repetition frequency and wavelength .lambda. to input
to a nonlinear optical crystal; a memory section in which a
plurality of the data groups is stored; a calculating section for
calculating the peak power density at which the maximum conversion
efficiency is obtained using the plurality of data groups stored in
the memory section; and an output section for outputting the peak
power density at which the maximum conversion efficiency can be
obtained that is calculated by the calculating section.
[0043] The present invention further provides a program for
activating a computer comprising: an input section for receiving
preset values comprising the repetition frequency Rep and constants
.alpha. and .beta. when obtaining radiated light of wavelength
{fraction (1/2)} .lambda. by causing light of wavelength .lambda.
to input to a nonlinear optical crystal; a calculating section for
calculating the optimal peak power density based on the following
Formula (1) using the preset values input into the input section;
and an output section for outputting the optimal peak power density
obtained by the calculating section.
Pc=.alpha..multidot.Rep.sup..beta. (1)
[0044] (Where: Rep=repetition frequency, and .alpha. and .beta. are
constants.)
[0045] The present invention also provides a computer readable
medium for holding a program which activates a computer comprising
an input section for receiving a data group comprising the peak
power density and conversion efficiency of incident light when
radiated light of wavelength of {fraction (1/2)} .lambda. is
obtained by causing incident light of a prescribed repetition
frequency and wavelength .lambda. to input to a nonlinear optical
crystal; a memory section for storing a plurality of the data
groups; a calculating section for calculating the peak power
density at which the maximum conversion efficiency is obtained
using the plurality of data groups stored in the memory section;
and an output section for outputting the peak power density at
which the maximum conversion efficiency can be obtained that is
calculated by the calculating section.
[0046] The present invention also provides a computer readable
medium for holding a program which activates a computer comprising:
an input section for receiving preset values comprising the
repetition frequency Rep and constants .alpha. and .beta. when
radiated light of wavelength of {fraction (1/2)} .lambda. is
obtained by causing incident light of wavelength .lambda. to input
to a nonlinear optical crystal; a calculating section for
calculating the optimal peak power density based on the following
Formula (1) using the preset values input into the input section;
and an output section for outputting the optimal peak power density
obtained by the calculating section.
Pc=.alpha..multidot.Rep.sup..beta. (1)
[0047] (Where: Rep=repetition frequency, and .alpha. and .beta. are
constants.)
[0048] Note that a variety of media, such as hard disk, flexible
disk, CD-ROM, semiconductor memory, DVD, etc., may be employed as
the computer readable medium employed in this invention.
[0049] In order to resolve the above-described first problem, the
present invention provides an optical wavelength converting method
which employs light from a laser oscillator that oscillates
coherent light of inherent wavelength .lambda. as incident light,
inputs the light to a nonlinear optical crystal and radiates out
light of wavelength {fraction (1/2)} .lambda., wherein the
nonlinear optical crystal is heated to and maintained at
200-600.degree. C.
[0050] The present invention further provides an optical wavelength
converting system comprising: a laser oscillator that oscillates
coherent light of inherent wavelength .lambda.; a nonlinear optical
crystal in which light from the laser oscillator is employed as
incident light, and light of wavelength {fraction (1/2)} .lambda.
is radiated; and a heating section that heats and maintains the
nonlinear optical crystal at 200-600.degree. C.
[0051] In the respective inventions as described above, it is even
more desirable that the heating and maintaining temperature is in
the range of 200-400.degree. C.
[0052] The desirable range for the wavelength of the incident light
in the respective inventions described above is 1000 nm or less,
but preferably 400-800 nm, more preferably 400-600 nm, and most
preferably 480-540 nm.
[0053] It is desirable that beam spreading of the incident light in
the respective inventions described above be 10 m rad or less, and
more preferably 0.3-4 m rad.
[0054] It is preferable that the time pulse width is 100 n sec or
less, and more preferably in the range of 1.times.10.sup.-3-80 n
sec.
[0055] It is desirable that the peak power density be 1 MW/cm.sup.2
or greater.
[0056] In order to resolve the aforementioned first problem, the
present invention provides an optical wavelength converting method
in which light from a laser oscillator, that oscillates coherent
light of inherent wavelength .lambda. as incident light, is input
to a lithium tetraborate (Li.sub.2B.sub.4O.sub.7) single crystal,
and light of wavelength {fraction (1/2)} .lambda. is radiated;
wherein the lithium tetraborate (Li.sub.2B.sub.4O.sub.7) single
crystal is heated to and maintained at 50-600.degree. C.
[0057] The present invention further provides an optical wavelength
converting system comprising: a laser oscillator that oscillates
coherent light of an inherent wavelength .lambda.; a lithium
tetraborate (Li.sub.2B.sub.4O.sub.7) single crystal that employs
light from the laser oscillator as incident light and radiates
light of wavelength {fraction (1/2)} .lambda.; and a heating
section that heats and maintains the lithium tetraborate single
crystal at 50-600.degree. C.
[0058] In the respective inventions, it is even more desirable that
the heating and maintaining temperature is in the range of
100-400.degree. C.
[0059] The desirable range for the wavelength of the incident light
in the respective inventions described above is 1000 nm or less,
but preferably 400-800 nm, more preferably 400-600 nm, and most
preferably 480-540 nm.
[0060] Further, in the respective inventions described above, beam
spreading of the incident light is 10 m rad or less, and more
preferably 0.3-4 m rad.
[0061] It is preferable that the time pulse width is 100 n sec or
less, and more preferably 1.times.10.sup.-3-80 n sec.
[0062] It is desirable that the peak power density be 1 MW/cm.sup.2
or greater.
[0063] In order to resolve the above-described second problem, the
present invention provides an optical wavelength converting method
in which fundamental waves of a prescribed wavelength and time
pulse width are input to a first nonlinear optical crystal and a
second nonlinear optical crystal sequentially, and the second
harmonic of the fundamental waves is generated, wherein the bulk
damage threshold of the first nonlinear optical crystal with
respect to the fundamental wave is larger than that of the second
linear optical crystal, and the effective nonlinear constant of the
second nonlinear crystal with respect to the fundamental waves is
larger than that of the first linear optical crystal.
[0064] The present invention further provides an optical wavelength
converting method in which fundamental waves of a prescribed
wavelength and time pulse width and the second harmonic wave of the
fundamental waves are input to a first nonlinear optical crystal
and a second nonlinear optical crystal sequentially, and the third
harmonic of the fundamental waves is generated, wherein the bulk
damage threshold of the first nonlinear optical crystal with
respect to the second harmonic wave is larger than that of the
second linear optical crystal, and the effective nonlinear constant
of the second nonlinear optical crystal with respect to the third
harmonic generation of the fundamental wave is larger than that of
the first linear optical crystal.
[0065] The present invention provides an optical wavelength
converting method in which a first fundamental wave of a prescribed
wavelength and time pulse width and a second fundamental wave of a
prescribed wavelength and time pulse width are input to a first
nonlinear optical crystal and a second nonlinear optical crystal
sequentially, and the sum frequency wave of the first fundamental
wave and the second fundamental wave are generated, wherein the
bulk damage threshold of the first nonlinear optical crystal with
respect to the first fundamental wave is larger than that of the
second linear optical crystal, and the effective nonlinear constant
of the second nonlinear optical crystal with respect to the sum
frequency wave generation from the first fundamental wave and the
second fundamental wave is larger than that of the first linear
optical crystal.
[0066] The present invention further provides a optical wavelength
converting system comprising: a first nonlinear optical crystal to
which a fundamental wave of a prescribed wavelength and time pulse
width is input and a second harmonic wave is generated, and a
second nonlinear optical crystal to which radiated light from the
first nonlinear optical crystal is input and a second harmonic wave
of the fundamental wave is generated, wherein the bulk damage
threshold of the first nonlinear optical crystal with respect to
the fundamental wave is larger than that of the second linear
optical crystal, and the effective nonlinear constant of the second
nonlinear optical crystal with respect to the second harmonic wave
generation of the fundamental wave is larger than that of the first
linear optical crystal.
[0067] The present invention further provides an optical wavelength
converting system comprising: a first nonlinear optical crystal to
which a fundamental wave of a prescribed wavelength and time pulse
width and the second harmonic of the fundamental wave are input,
and a third harmonic wave is generated; and a second nonlinear
optical crystal to which radiated light from the first nonlinear
optical crystal is input and the third harmonic wave is generated;
wherein the bulk loss threshold of the first nonlinear optical
crystal with respect to the second harmonic wave is larger than
that of the second linear optical crystal, and the effective
nonlinear constant of the second nonlinear optical crystal with
respect to the third harmonic wave generation of the fundamental
wave is larger than that of the first linear optical crystal.
[0068] The present invention further provides an optical wavelength
converting system comprising: a first nonlinear optical crystal to
which a first fundamental wave of a prescribed wavelength and time
pulse width and a second fundamental wave of a prescribed
wavelength and time pulse width are input, and the sum frequency
wave of the first fundamental wave and the second fundamental wave
is generated; and a second nonlinear optical crystal to which
radiated light from the first nonlinear optical crystal is input
and the sum frequency wave is generated; wherein the bulk damage
threshold of the first nonlinear optical crystal with respect to
the first fundamental wave is larger than that of the second linear
optical crystal, and the effective nonlinear constant of the second
nonlinear optical crystal with respect to the sum frequency wave
generation from the first fundamental wave and the second
fundamental wave is larger than that of the first linear optical
crystal.
[0069] The present invention provides a laser oscillating system
comprising: a fundamental wave oscillator for oscillating a
fundamental wave of a prescribed wavelength and time pulse width;
and an optical wavelength converting system to which the
fundamental wave from the fundamental wave oscillator is input and
a second harmonic wave is generated; wherein the optical wavelength
converting system is the optical wavelength converting system
according to the present invention.
[0070] The present invention provides a laser oscillating system
comprising: a fundamental wave oscillator for oscillating a
fundamental wave of a prescribed wavelength and time pulse width
and the second harmonic of this fundamental wave; and an optical
wavelength converting system to which the fundamental wave and the
second harmonic wave from the fundamental wave oscillator are
input, and a third harmonic wave is generated; wherein the optical
wavelength converting system is the optical wavelength converting
system according to the present invention.
[0071] The present invention provides a laser oscillating system
comprising: a fundamental wave oscillator for oscillating a first
fundamental wave of a prescribed wavelength and time pulse width
and a second fundamental wave of a prescribed wavelength and time
pulse width; and an optical wavelength converting system to which
the first fundamental wave and the second fundamental wave from the
fundamental wave oscillator are input and a sum frequency wave is
generated, wherein the optical wavelength converting system is the
optical wavelength converting system according to the present
invention.
[0072] In the respective inventions described above, even if the
conversion efficiency of the first nonlinear optical crystal is
low, fundamental waves, etc., that have passed through can be
converted with high efficiency by the second nonlinear optical
crystal. In addition, even if the second nonlinear optical crystal
has low resistance to light damage, incident light passes through
the first nonlinear coefficient which is highly resistance to light
damage. As a result, the power to which the second nonlinear
optical crystal is subjected is decreased, making it possible to
input the light thereto.
[0073] In other words, by combining different types of nonlinear
optical crystals that have specific relationships to one another,
the deficits of the different crystals are mutually compensated, so
that a high conversion efficiency and high resistance to light
damage can be achieved. Accordingly, high power sum frequency waves
such as the second harmonic wave can be obtained with good
efficiency.
[0074] It is desirable that the first nonlinear optical crystal in
the respective inventions described above be a lithium tetraborate
(Li.sub.2B.sub.4O.sub.7) single crystal. This is because, although
the conversion efficiency of LB4 (Li.sub.2B.sub.4O.sub.7) is
relatively low, it has an extremely excellent resistance to damage
from light.
[0075] When LB4 is employed as the first nonlinear optical crystal,
it is desirable that the second nonlinear optical crystal be
LiB.sub.3O.sub.5 (LBO), CsLiB.sub.6O.sub.10 (CLBO), KTiOPO.sub.4
(KTP), or .beta.-BaB.sub.2O.sub.4 (BBO).
[0076] The technical significance of the present invention will now
be explained with reference to experimental results.
[0077] The present inventors used experiments to determine the
relationship between the peak power density and the conversion
efficiency of the incident light for an LB4 crystal, which is a
nonlinear optical crystal. These results are shown in FIG. 1. The
conditions for the LB4 crystal and the oscillator of the incident
light employed in the experiments were as follows.
[0078] A combination of an Nd:YAG laser and an LBO crystal,
employed as an SHG element, was used for the incident light
oscillator. In other words, the light which is input to the LB4
crystal was green laser (532 nm) which is the second harmonic wave
of the near infrared light (1064 nm) from the Nd:YAG laser. Note
that different oscillators were employed to obtain incident light
with a repetition frequency of 5 kHz or greater, and incident light
with a repetition frequency of 100 Hz or less.
[0079] The peak power density is obtained by dividing the average
output of the incident light by the repetition frequency, beam area
and time pulse width. Therefore, in these experiments, the average
output of the incident light was adjusted by adjusting the power of
the excitation light power applied to the YAG laser. In addition, a
condensing lens was used to adjust the beam diameter (beam
area).
[0080] The LB4 crystal employed had a crystal length of either 35
mm or 60 mm. Note that the cross-sectional area of the LB4 crystal
does not affect the conversion efficiency, however, in the main, an
LB4 crystal of cross-section 15 mm.times.15 mm was employed.
[0081] In FIG. 1, symbol XI (indicated by .diamond-solid.) is data
for the case of a crystal length of 35 mm, repetition frequency of
1 Hz, and beam diameter of 5.5 mm.
[0082] Symbol X.sub.10 (indicated by .box-solid.) is data for the
case of a crystal length of 35 mm, repetition frequency of 10 Hz,
and beam diameter of 5.5 mm or 11 mm (11 mm for a peak power
density of less than 200 MW/cm.sup.2; 5.5 mm for a peak power
density of 200 MW/cm.sup.2 or more:).
[0083] Symbol X.sub.100 (indicated by .tangle-solidup.) indicates
data for the case of a crystal length of 35 mm, repetition
frequency of 100 Hz, and beam diameter of 5.5 mm or 11 mm (11 mm
for a peak power density of less than 100 MW/cm.sup.2; 5.5 mm for a
peak power density of 100 MW/cm.sup.2 or more).
[0084] Symbol Y.sub.10 (indicated by .quadrature.) indicates data
for the case of a crystal length of 60 mm, repetition frequency of
10 Hz, and beam diameter of 11 mm.
[0085] Symbol Y.sub.100 (.DELTA.) indicates data for the case of a
crystal length of 60 mm, repetition frequency of 100 Hz, and beam
diameter of 11 mm.
[0086] The time pulse width when the above data was obtained was
fixed at 3 n sec, and the peak power density was adjusted by
varying the average output within the range of 0-26 W. Note that
beam spreading was about 1 m rad when the beam diameter was 5.5 mm,
and about 0.5 m rad when the beam diameter was 11 mm.
[0087] The symbol Z.sub.5 (indicated by .circle-solid.) indicates
data in the case of a crystal length of 35 mm and repetition
frequency of 5 kHz. When this data was obtained, the time pulse
width was fixed at 25 n sec, the average output was fixed at 30 W,
and the peak power density was adjusted by varying the beam
diameter in the range of 0.4-1.0 mm. Note that beam spreading was
around several rad (5 m rad or less).
[0088] The symbol Z.sub.10 (.diamond.) indicates data in the case
of a crystal length of 35 mm and repetition of frequency 10 kHz.
When this data was obtained, the time pulse width was fixed at 30 n
sec, the average output was fixed at 30 W, and the peak power
density was adjusted by varying the beam diameter in the range of
0.4-1.0 mm. Note that beam spreading was around several rad (5 m
rad or less).
[0089] In general, as shown in the following Formula (2), it is
known that the more that the peak power P of the incident light
increases, the more the conversion efficiency .eta. increases.
.eta.=a.multidot.tan h.sup.2(b.multidot.P.sup.0.5) (2)
[0090] (Where a and b are constants determined mainly according to
the crystal type and crystal length.)
[0091] As shown in FIG. 1, when light from the same oscillator is
input to an LB4 crystal with a crystal length of 35 mm, the data
points indicated by X.sub.1, X.sub.10, and X.sub.100 coincide at
incident light peak power densities of 100 MW/cm.sup.2 or less,
regardless of the repetition frequency. Thus, by examining data
only within this range, it is predicted that the curve indicated by
symbol X.sub.0 can be traced out in accordance with the principle
shown in Formula (2) as the peak power density increases. Note that
when a and b on the curve indicated by symbol X.sub.0 are
determined from data in this range, then a=32 and b=0.085.
[0092] As shown by data points indicated by Z.sub.5, Z.sub.10, when
different oscillators are employed, then, even in the case of the
same crystal length of 35 mm, a deviation from curve X.sub.0 can be
seen over the entire peak power density range. At low peak power
densities, however, a rising curve with roughly the same slope as
X.sub.0 could be obtained. Note that the large amount of beam
spreading is thought to be the main cause of the curve
deviations.
[0093] Similarly, in the case of an LB4 crystal having a crystal
length of 60 mm, the data points indicated by Y.sub.10, Y.sub.100
coincide at incident light peak power densities of 50 MW/cm.sup.2
or less, regardless of the repetition frequency. Thus, by examining
data only within this range, it is predicted that the curve
indicated by symbol Y.sub.0 can be traced out in accordance with
the principle shown in Formula (2) even if the peak power density
increases. Note that when a and b on the curve indicated by symbol
Y.sub.0 are determined from data in this range, then a=22 and
b=0.18.
[0094] However, once the peak power density of the incident light
exceeded a set value, then the data points indicated by X.sub.10,
X.sub.100, Y.sub.100, Z.sub.5 and Z.sub.10 deviated from the ideal
curves X.sub.0, Y.sub.0 predicted by Formula (2), and a
deteriorating conversion efficiency phenomenon was observed in this
experiment.
[0095] In addition, it also became clear from this data that the
higher the repetition frequency, the lower the peak power density
at which the conversion efficiency began to decrease.
[0096] In addition, another trend seen was that the longer the
crystal length, the higher the conversion efficiency became.
[0097] As described above, these experiments investigated
conversion efficiency while varying the peak power density of the
incident light, and examined the stability of the radiated light.
As a result, a two photon absorption phenomenon in which the output
of radiated light becomes unstable, was discovered at the point
where separation from curves X.sub.0, Y.sub.0 that are in
accordance with Formula (2) occurs, this point being exactly where
the conversion efficiency starts to decrease. This phenomenon of
destabilization of the radiated light was not observed at all prior
to the point where the conversion efficiency began to decrease.
Moreover, once the conversion efficiency began to decrease, then
this phenomenon became more marked as the peak power density was
increased further.
[0098] In other words, the present inventors discovered that the
peak power density that provides the maximum conversion efficiency
when the repetition frequency of the laser oscillator and the
length of the LB4 crystal are held constant corresponds to "an
optimal value for the peak power density of the incident light at
which the maximum radiated light output is obtained without
essentially giving rise to the so-called two photon absorption
phenomenon that leads to destabilization of output" (hereinafter,
referred to simply as "optimal peak power density").
[0099] Next, based on data X.sub.1, X.sub.10, X.sub.100 in FIG. 1,
a study was made of how the optimal peak power density varies in
response to repetition frequency for the case of a crystal length
of 35 mm. As shown in FIG. 2, a nearly straight line graph was
obtained. The repetition frequency of the incident light is plotted
in logarithmic scale along the horizontal axis, and the peak power
density of the incident light is plotted in logarithmic scale along
the vertical axis in FIG. 2.
[0100] The above Formula (1) was used to obtain the straight line
equation for this optimal peak power density Pc.
Pc=.alpha..multidot.Rep.sup..beta. (1)
[0101] (Where: Rep=repetition frequency, and .alpha. and .beta. are
constants.)
[0102] The constants .alpha. and .beta. are determined mainly
according to crystal length and the type of crystal. In the case of
the LB4 crystal having a crystal length of 35 mm shown in FIG. 2,
.alpha.=576 and .alpha.=-0.27. For an LB4 crystal having a crystal
length of 60 mm, a=154 and .beta.=-0.25.
[0103] Note that as shown by data points Z.sub.5 and Z.sub.10,
there is an effect on the conversion efficiency with changes in the
beam spreading of the incident light. This is because a loss of
conversion efficiency occurs when beam spreading of the incident
light exceeds the tolerance angle range that is determined based on
the phase matching conditions of the LB4 crystal.
[0104] However, in this case as well, by correcting the effects of
beam spreading, it is possible to determine the optimal peak power
density by theoretically evaluating the conversion characteristics.
From the experiments it was confirmed that when beam spreading of
the incident light exceeds 10 m rad, then conversion efficiency
falls to {fraction (1/10)} of its value as compared to when beam
spreading of the incident light is 1 m rad. Thus, it is difficult
to obtain radiated light of a level that can be practically
employed.
[0105] Thus, it is most desirable that the optimal peak power
density be employed for the peak power density of the incident
light. However, from a practical perspective, it is possible to
employ a set range of peak power densities, with the optimal peak
power density taken as the standard.
[0106] In other words, it is desirable to set the peak power
density of the incident light to be equal to or less than the
optimal peak power density. This is because when the peak power
density is greater than the optimal peak power density, the output
of the radiated light becomes unstable. However, two photon
absorption becomes gradually more marked once the optimal peak
power is exceeded, rather than having a large effect immediately.
Thus, if the peak power density of the incident light is made to be
10-fold or less than the optimal peak power density, no hindrance
to practical application is incurred. Further, if the peak power
density of the incident light is made to be two-fold or less than
the optimal peak power density, then it is even more possible to
control output destabilization.
[0107] In order to obtain the highest radiated light power possible
with good efficiency, it is necessary to set the peak power density
to 0.1 fold or more than the optimal peak power density, with 0.5
fold or greater being desirable.
[0108] Note that if consideration is given to the lifespan of a
nonlinear crystal, it is desirable to set the incident light peak
power density to be 0.8 fold or less than the optimal peak power
density. Accordingly, the most desirable peak power density for the
incident light is in the range of 0.5-0.8 fold greater than the
optimal peak power density.
[0109] In addition, taking the optimal peak power density as a
boundary, the phenomenon in which conversion efficiency decreases
and output becomes unstable is observed most remarkably as the
wavelength becomes shorter, and in particular when converting from
so-called green light to ultraviolet light. Accordingly, the
present invention is particularly effective at incident light
wavelengths of 1000 nm or less, with incident light wavelengths in
the range of 400-800 nm being desirable, and in the range of
400-600 nm being even more desirable.
[0110] As a result of further investigations by the present
inventors, it was discovered that this optimal peak power density
could be increased by heating and maintaining the nonlinear optical
crystal to 50.degree. C. or higher. As discussed above, it has been
the conventional practice to heat and maintain nonlinear optical
crystals for the purpose of protecting them from moisture, or to
carry out phase matching using temperature. In these cases,
however, the degree of heating was around less than 200.degree. C.
In other words, heating and maintaining to temperatures of
200.degree. C. or higher as in the present invention was not
performed.
[0111] The effects of heating to a relatively high temperature as
in the present invention will be explained using Table 1 and FIG.
3, and Table 2 and FIG. 4.
[0112] Conversion efficiency was examined for the case where the
incident light's average repetition frequency, beam diameter and
time pulse width were fixed at 10 kHz, 0.25 mm and 28 n sec,
respectively, and only the average output of the incident light was
varied. These results are shown in Table 1. FIG. 3 is a graph
obtained from the data in Table 1 by plotting the average output of
the incident light on the horizontal axis and the conversion
efficiency on the vertical axis.
[0113] Similarly, the conversion efficiency was examined for the
case where the incident light's average repetition frequency, beam
diameter and time pulse width were fixed at 10 kHz, 0.35 mm and 28
n sec, respectively, and only the average output of the incident
light was varied. These results are shown in Table 2. FIG. 4 is a
graph obtained from the data in Table 1 by plotting the average
output of the incident light on the horizontal axis and the
conversion efficiency on the vertical axis.
[0114] Note that the average output of the incident light is in
proportion to the peak power density of the incident light as shown
in Table 2. In addition, the temperature (Temp) in the tables and
figures is the heating and maintaining temperature of LB4 (RT is
room temperature, approximately 25.degree. C.).
1 TABLE 1 Temp: RT Temp: 60.degree. C. Temp: 100.degree. C. Temp:
150.degree. C. Temp: 200.degree. C. Incident Radiated Conversion
Radiated Conversion Radiated Conversion Radiated Conversion
Radiated Conversion light light efficiency light efficiency light
efficiency light efficiency light efficiency [W] [W] [%] [W] [%]
[W] [%] [W] [%] [W] [%] 5.51 0.09 1.63 0.12 2.18 0.13 2.36 0.15
2.72 0.15 2.72 6.69 0.16 2.39 7.92 0.25 3.16 0.29 3.66 0.32 4.04
0.33 4.17 0.33 4.17 9.16 0.36 3.93 10.41 0.46 4.42 0.53 5.09 0.57
5.48 0.6 5.77 0.63 6.05 11.64 0.54 4.64 0.66 5.67 0.72 6.18 0.76
6.53 0.78 6.70 12.85 0.73 5.68 0.83 6.46 0.87 6.77 0.92 7.16 1 7.78
14.03 0.9 6.42 0.99 7.06 1.06 7.56 1.13 8.06 1.2 8.55 15.16 0.81
5.35 1.16 7.65 1.26 8.31 1.32 8.71 1.43 9.43 16.25 0.66 4.06 1.33
8.18 1.42 8.74 1.52 9.35 1.6 9.85 17.30 1.5 8.67 1.6 9.25 1.71 9.89
1.85 10.70 18.30 1.22 6.67 1.82 9.94 2 10.93 2.17 11.86 19.28 1.99
10.32 2.3 11.93 2.37 12.29 20.25 2.54 12.54 21.23 2.74 12.91 22.25
3.05 13.71
[0115]
2TABLE 2 Incident Light Peak power Temp: RT Temp: 100.degree. C.
Temp: 200.degree. C. Temp: 300.degree. C. Temp: 385.degree. C.
Average density Radiated Conversion Radiated Conversion Radiated
Conversion Radiated Conversion Radiated Conversion output [MW/
light efficiency light efficiency light efficiency light efficiency
light efficiency [W] cm.sup.2] [W] [%] [W] [%] [W] [%] [W] [%] [W]
[%] 3.7 13.2 0.06 1.63 0.06 1.63 0.08 2.18 0.09 2.45 0.08 2.18 4.9
17.6 6.1 21.9 0.16 2.62 0.18 2.95 0.20 3.28 0.22 3.61 0.21 3.44 7.5
26.9 8.7 31.0 0.33 3.82 0.36 4.16 0.39 4.51 0.45 5.20 0.45 5.20 9.9
35.5 10.8 38.5 0.51 4.74 0.56 5.21 0.61 5.67 0.67 6.23 0.70 6.51
11.8 42.1 0.62 5.28 0.67 5.70 0.74 6.30 0.81 6.89 0.84 7.15 12.9
46.0 0.73 5.68 0.81 6.30 0.90 7.00 1.00 7.78 1.03 8.02 13.9 49.8
0.81 5.83 0.95 6.83 1.09 7.84 1.18 8.49 1.23 8.85 14.9 53.2 0.92
6.20 1.09 7.34 1.22 8.22 1.36 9.16 1.40 9.43 15.8 56.6 1.11 7.03
1.30 8.23 1.45 9.18 1.59 10.06 1.67 10.57 16.5 59.1 0.87 5.27 1.46
8.85 1.67 10.12 1.79 10.85 1.91 11.58 17.1 61.3 1.54 9.01 1.77
10.35 1.94 11.35 2.04 11.93 17.8 63.8 1.66 9.33 1.91 10.73 2.05
11.52 2.18 12.25 18.8 67.4 1.88 10.00 2.14 11.38 2.36 12.55 2.50
13.30 19.8 70.8 2.45 12.41 2.70 13.67 2.83 14.33
[0116] As is clear from Table 1 and FIG. 3, at room temperature,
the peak power density reaches an optimal value at an incident
light output of approximately 14 W. In contrast, in LB4 that is
heated to and maintained at 60.degree. C., the peak power density
reaches an optimal value at an incident light output of around 17
W. Further, it may be understood from this data that as the heating
and maintenance temperature is further increased, the optimal peak
power density continues to rise and a local maximum value for the
conversion efficiency within the measured range is not
observed.
[0117] As is clear from Table 2 and FIG. 4, at room temperature,
the peak power density reaches its optimal value (of approximately
57 MW/cm.sup.2) at an incident light output of approximately 16 W.
In contrast, in the case of LB4 that is heated to and maintained at
100.degree. C. or more, the optimal peak power density increases
further without observing a local maximum value for the conversion
efficiency within the measured range. Moreover, the higher the
heating and maintaining temperature for LB4 becomes, the more the
conversion efficiency increases.
[0118] In this way, not only does heating reduce the impact of heat
release due to two photon absorption, which causes change in the
refractive index, but it also increases the optimal peak power
density. In other words, it was discovered that the peak power
density of the incident light at which output could be stably
obtained could be increased, without substantially causing the
phenomenon of output destabilization from two photon
absorption.
[0119] The higher the heating and maintenance temperature, the
greater the effects obtained. However, a temperature 200.degree. C.
or more is required. As a result, it is possible to reduce the
impact of two photon absorption, and eliminate the decrease in
conversion efficiency, while removing the phenomenon of a
decreasing conversion efficiency and enabling provision of a high
output stably.
[0120] On the other hand, it is not desirable to increase the
heating and maintenance temperature to greater than 600.degree. C.
When the temperature exceeds 600.degree. C., the heat insulating
section for preventing outflow of heat around the heating section
becomes too large and impractical from the standpoint of practical
use.
[0121] Furthermore, it is desirable that the heating and
maintenance temperature be 400.degree. C. or less. This is because
temperatures above 400.degree. C. yield little improvement in the
effect of diminishing the impact of two photon absorption, so that
only a small benefit is conferred from these higher
temperatures.
[0122] Note that conversion efficiency decreases when beam
spreading of the incident light exceeds the tolerance angle zone
that is determined based on the phase matching conditions for the
LB4 crystal. Accordingly, desirable beam spreading for the incident
light is 10 m rad or less, and more desirably in the range of 0.3-4
m rads.
[0123] It is also preferable that the time pulse width be 100 n sec
or less, and more preferably in the range of 1.times.10.sup.-3-80 n
sec.
[0124] In general, the higher the repetition frequency, the more
the pulse width broadens and the smaller the pulse energy becomes.
Conversely, at lower repetition frequencies, the pulse width can be
narrowed and the pulse energy increased. For this reason, an upper
limit is determined for the range in which the desired peak density
can be obtained.
[0125] Further, it is desirable that the peak power density of the
incident light be 1 MW/cm.sup.2 or more. Note that the upper limit
for the peak power density of the incident light is that at which
bulk damage to the crystal (dielectric breakdown), or damage to the
coating film or the end faces of the crystal, does not occur.
BRIEF DESCRIPTION OF DRAWINGS
[0126] FIG. 1 is a graph showing the relationship between the peak
power density of the incident light and conversion efficiency.
[0127] FIG. 2 is a graph examining how the optimal peak power
density changes in response to repetition frequency.
[0128] FIG. 3 is a graph showing the results of an examination of
the relationship between conversion efficiency and average output
of the incident light in response to the heating temperature of the
LB4 crystal.
[0129] FIG. 4 is a graph showing the results of an examination of
the relationship between conversion efficiency and average output
of the incident light in response to the heating temperature of the
LB4 crystal.
[0130] FIG. 5 is a structural view showing a first embodiment of
the ultraviolet laser oscillator employing the optical wavelength
converting method according to the present invention.
[0131] FIG. 6 is a structural view of an example of the computer
system that activates functions according to the program of the
present invention.
[0132] FIG. 7 is structural view of another example of the computer
system which activates functions according to the program of the
present invention.
[0133] FIG. 8 is a structural view showing the laser oscillating
system according to another embodiment of the present
invention.
[0134] FIG. 9 is a graph showing how the peak power density varies
using the first crystal.
BEST MODE FOR CARRYING OUT THE INVENTION
[0135] Preferred embodiments of the present invention will now be
explained with reference to the figures. Note, however, that the
present invention is not limited thereto.
[0136] FIG. 5 is a structural view showing an embodiment of the
ultraviolet laser oscillator employing the optical wavelength
converting method according to the present invention. The
ultraviolet laser oscillator shown in FIG. 5 is composed of a green
laser oscillator 10 and a wavelength converting system 20.
[0137] Green laser oscillator 10 is composed of a main oscillator
11 comprising an Nd:YAG laser, and a converter 12 that converts the
fundamental wave (1064 nm) output from main oscillator 11 into
second harmonic waves, that is, green light (532 nm).
[0138] Wavelength converting system 20 is composed of separators 21
and 22 for separating the green light radiated from converter 12
from fundamental waves that have passed through converter 12
without being wavelength converted; an LB4 crystal box 23 to which
the green light separated by separators 21 and 22 is input as the
incident light; prism 24 which separates the radiated light
outgoing from LB4 crystal box 23; and beam damper 25 for absorbing
the fundamental waves separated by separator 21.
[0139] A lithium tetraborate single crystal LB4 is disposed inside
LB4 crystal box 23 in a manner so as to satisfy the phase matching
angle. Further, a heating device is also housed inside LB4 crystal
box 23 for heating and maintaining the temperature of this LB4
crystal at 600.+-.1.degree. C.
[0140] In the ultraviolet laser oscillator according to the present
embodiment, green light is converted by LB4 crystal box 23 into
ultraviolet light (266 nm), which is the second harmonic waves of
the green light and the fourth harmonic waves of the fundamental
wave. Further, using prism 24, it is possible to extract only the
wavelength converted ultraviolet light.
[0141] The optimal peak power density of LB4 crystal box 23 attains
a higher value than if it is employed at room temperature without
heating. The peak power density of the green light that is input to
LB4 crystal box 23 from separator 22 is 0.5-2 times greater than
this optimal peak power density.
[0142] The present embodiment employs incident light that increases
the optimal peak power density, and that has a peak power density
that is less than or equal to but approaching this optimal peak
power density. For this reason, even if the peak power density of
the incident light is increased, it is possible to obtain a stable
output. Accordingly, high conversion efficiency can be stably
reached using a lithium tetraborate single crystal LB4, to achieve
an all solid state ultraviolet laser oscillator that is durable
with respect to practical application.
[0143] FIG. 6 is a structural view of an embodiment of the computer
system for achieving functions according to the program of the
present invention. In FIG. 6, reference symbol 31 is a calculator,
32 is an input device, 33 is a memory for storing data input from
input device 32, 34 is a display for showing the result calculated
by calculator 31, and 35 is a printer for printing out the result
calculated by calculator 31.
[0144] In the computer system of the present embodiment, the
following steps can be employed to obtain the optimal peak power
density in a wavelength converting system in which radiated light
of wavelength {fraction (1/2)} .lambda. is obtained by causing
light of wavelength .lambda. to input to a nonlinear optical
crystal under conditions of a prescribed repetition frequency and
crystal length.
[0145] First, a data group comprising the peak power density and
the conversion efficiency of the incident light is input from input
device 32. The input process may be performed manually, or a
transmission signal from a measuring device that measures the
conversion efficiency may directly input the data, without
employing a manual input step. A plurality of these input data
groups are then stored in memory 33.
[0146] Next, the calculator 31 extracts the maximum conversion
efficiency from among the conversion efficiency data in the
multiple data groups that were stored in memory 33. The peak power
density that provides this maximum conversion efficiency is then
determined. Note that it is also acceptable to extract the maximum
conversion efficiency and the peak power density at that time from
continuous data obtained using an approximation formula
corresponding to the data groups. As a result, the peak power
density that provides the maximum conversion efficiency can be
obtained without being effected by errors in individual conversion
efficiency datum.
[0147] The peak power density that provides the maximum conversion
efficiency that was determined by calculator 31, i.e., the optimal
peak power density, is displayed on display 34 as well as printed
out by printer 35.
[0148] According to the present embodiment, an operator operating a
wavelength converting system in which radiated light of wavelength
{fraction (1/2)} .lambda. is obtained by causing light of
wavelength .lambda. to input to a nonlinear optical crystal under
conditions of a prescribed repetition frequency and crystal length,
is able to set a suitable peak power density by referring to the
optimal peak power density which is displayed and printed out.
Further, if the maximum peak power density output is directly input
into the laser oscillator, then the peak power density of the
incident light can be automatically controlled.
[0149] FIG. 7 is a structural view of another example of the
computer system that activates functions according to the program
of the present invention. In FIG. 7, reference symbol 41 is a
calculator, 42 is an input device, 44 is a display for displaying
the result calculated by calculator 41, and 45 is a printer for
printing out the result calculated by the calculator 41.
[0150] In the computer system of the present embodiment, the
following steps can be employed to obtain the optimal peak power
density in a wavelength converting system in which radiated light
of wavelength {fraction (1/2)} .lambda. is obtained by causing
light of wavelength .lambda. to input to a nonlinear optical
crystal under conditions of a prescribed repetition frequency and
crystal length.
[0151] First, preset values comprising repetition frequency Rep and
constants .alpha. and .beta. are input manually from input device
42. Note that .alpha. and .beta. are determined in advance based on
experimental results.
[0152] Next, calculator 41 calculates the optimal peak power
density based on the following Formula (1)
Pc=.alpha..multidot.Rep.sup..beta. (1)
[0153] (Where: Rep=repetition frequency, and .alpha. and .beta. are
constants.)
[0154] The optimal peak power density determined by calculator 41
is displayed on display 44 and is printed out by printer 45.
[0155] According to the present embodiment, an operator operating a
wavelength converting system in which radiated light of wavelength
{fraction (1/2)} .lambda. is obtained by causing light of
wavelength .lambda. to input to a nonlinear optical crystal under
conditions of a prescribed repetition frequency and crystal length
is able to set a suitable peak power density by referring to the
optimal peak power density which is displayed and printed out.
Moreover, if the maximum peak power density output is directly
input into the laser oscillator, then the peak power density of the
incident light can be automatically controlled.
[0156] Another embodiment of the present invention will now be
explained with reference to FIGS. 8 and 9.
[0157] FIG. 8 is a structural view showing the laser oscillating
system according to this embodiment. The laser oscillating system
in FIG. 8 comprises a fundamental wave oscillator 50 and an optical
wavelength converting system 60.
[0158] Fundamental wave oscillator 50 may be composed of single
laser oscillator such as an Nd:YAG laser, or may be composed of a
laser oscillator and a converter that wavelength converts the light
oscillated by this laser oscillator.
[0159] Wavelength converting system 60 is composed of separators
61,62 for separating fundamental waves of wavelength .lambda.
radiated from fundamental wave oscillator 50 from light of other
wavelengths; a first crystal 63 to which fundamental waves
separated by separators 61 and 62 are input as incident light I0;
second crystal 64 to which radiated light I1 from first crystal 63
is input; prism 65 for separating radiated light I2 radiated from
second crystal 64; and beam damper 66 for absorbing light of
wavelengths other than the fundamental waves that was separated by
separator 61.
[0160] First crystal 63 and second crystal 64 are different types
of nonlinear optical crystals, and are disposed so as to satisfy
the phase matching angle with respect to fundamental wave
.lambda..
[0161] Applicable nonlinear optical crystals that can be employed
for first crystal 63 and second crystal 64 include, for example,
LB4 (Li.sub.2B.sub.4O.sub.7), KTP (KTiOPO.sub.4),
BBO(.beta.-BaB.sub.2O.sub.4- ), CLBO(CsLiB.sub.6O.sub.10), LBO
(LiB.sub.3O.sub.5), KDP (KH.sub.2PO.sub.4), and the like. First
crystal 63 and second crystal 64 are each selected from among these
nonlinear optical crystals after giving consideration to their
conversion efficiency and resistance to damage, as will be
explained below.
[0162] First, both crystals are selected so that first crystal 63
is provided with a higher resistance to damage from light than
second crystal 64. Specifically, when the bulk damage threshold
with respect to the fundamental wave is compared, the crystals are
selected to have a relationship such that first crystal 63 has a
larger bulk damage threshold than second crystal 64.
[0163] This bulk damage threshold is the threshold for the peak
power density of the incident light at which bulk damage occurs
(i.e., the peak power density of the peak value). Bulk damage is
the damage that occurs when incident light damages the chemical
bonds of the crystal.
[0164] The shorter the wavelength of the incident light, or the
longer the time pulse width becomes, the lower the bulk damage
threshold. However, the size relationship between the bulk damage
threshold values of different crystals at a given wavelength or
time pulse width does not vary, even when the wavelength and time
pulse width conditions are changed.
[0165] For example, the bulk damage threshold for the main
nonlinear optical crystals at a wavelength of 1064 nm and a time
pulse width of 1 n sec has the values as shown below. The size
relationship between these threshold values does not vary under
other conditions and is as shown below.
[0166] (Bulk Damage Threshold at Wavelength=1064 nm, Time Pulse
Width=1 n Sec)
[0167] LB4: .about.90 GW/cm.sup.2
[0168] LBO: .about.45 GW/cm.sup.2
[0169] CLBO: .about.26 GW/cm.sup.2
[0170] KDP: .about.14 GW/cm.sup.2
[0171] BBO: .about.13 GW/cm.sup.2
[0172] KTP: .about.0.6 GW/cm.sup.2
[0173] (Size Relationship Between Bulk Damage Threshold Values)
[0174] LB4>LBO>CLBO>KDP>BBO>KTP
[0175] In addition to bulk damage, another type of light damage is
surface damage occurring from the crystal surface. In general, the
bulk damage threshold is larger than the surface damage threshold.
Accordingly, the peak power density of the incident light that
causes crystal breakdown is usually determined based on the surface
damage. However, the surface damage threshold will change according
to how polished the surface is, the presence or absence of water
absorption, the extent to which the incident light is condensed,
etc., so that an objective comparison is difficult. Accordingly,
when evaluating the property of resistance to light damage, it is
appropriate to employ the bulk damage threshold.
[0176] Next, the crystals are selected so that second crystal 64
was provided with higher conversion efficiency than first crystal
63. Specifically, the crystals are selected according to a
relationship such that, when comparing the effective nonlinear
constant with respect to the fundamental wave, second crystal 64
has a larger effective nonlinear constant than first crystal
63.
[0177] The effective nonlinear constant is the effective conversion
coefficient calculated from the nonlinear constant of the nonlinear
optical crystal and the incident angle. The incident angle is
selected so that there is phase matching in accordance with the
wavelength of the incident light. Thus, provided that the
wavelength of the incident light is determined, then it is possible
to actually compare the effective nonlinear constant of each
crystal.
[0178] For example, the effective nonlinear constants at a
wavelength of 1064 nm for the main nonlinear optical crystals are
as follows, with the crystals having the size relationship shown
below with respect to this constant.
[0179] (Effective Nonlinear Constants and Phase Matching Angles at
1064 nm Wavelength)
3 LB4: 0.08 pm/V, 31.degree. LBO: 1.05 pm/V, 90.degree. (type I)
CLBO: 0.47 pm/V, 29.4.degree. (type I) 0.95 pm/V, 42.9.degree.
(type II) BBO: 1.64 pm/V, 22.9.degree. (type I) 1.25 pm/V,
33.1.degree. (type II) KDP: 0.27 pm/V, 41.2.degree. (type I) 0.34
pm/V, 59.2.degree. (type II) KTP: 3.24 pm/V, 90.degree. (type
II)
[0180] (Size Relationship Between Effective Nonlinear Constants at
1064 nm Wavelength)
[0181] KTP>BBO>LBO>CLBO>KDP>LB4
[0182] When selecting LB4, which has the largest bulk damage
threshold, for use as first crystal 63, there are various nonlinear
crystals that can be selected for use as second crystal 64.
However, the incident light wavelength range that is employed must
be limited to a range in which the effective nonlinear constant of
second crystal 64 becomes larger than the effective nonlinear
constant of first crystal 63 (LB4). The wavelength ranges in which
the effective nonlinear constant of the various crystals becomes
larger than the effective nonlinear constant of LB4 is as
follows.
[0183] (Incident Light Wavelength Range for Obtaining Effective
Nonlinear Constant Larger than that of LB4)
[0184] LBO: 2000-500 nm
[0185] CLBO: 2000-472 nm
[0186] BBO: 1400-409 nm
[0187] KDP: 1300-500 nm
[0188] KTP: 2000-990 nm
[0189] In the laser oscillating system according to the present
invention, incident light I0, which is a fundamental wave of
wavelength .lambda., is input to first crystal 63. Radiated light
I1 from first crystal 63 is composed of the second harmonic wave,
wavelength .lambda./2, and fundamental waves of wavelength .lambda.
that passed through first crystal 63 without being converted. Here,
since a portion of incident light I0 is converted into the second
harmonic wave, the peak power density of the fundamental waves that
are included in radiated light I1 has become smaller than the peak
power density of the incident light I0. For this reason, it is
possible to set the peak power density of the incident light I0 to
a higher value than in the case where light is directly input to
second crystal 64.
[0190] In this case, as shown in FIG. 9, the effect of protecting
second crystal 64 with first crystal 63 is not limited to the range
of the conversion efficiency of first crystal 63. Namely, as shown
by the solid line in FIG. 9, incident light I0 is distributed
within the beam diameter range such that the maximum peak power
density occurs at the beam center. Light near the beam center where
a high peak power density is obtained is most apt to cause damage
to the crystal. On one hand, this light near the beam center having
this high peak power density is most easily converted. For this
reason, the peak power density of the fundamental waves included in
radiated light I1 decreases greatly near the beam center, as shown
by the wavy line in FIG. 9. Accordingly, it is possible to greatly
reduce the impact on second crystal 64.
[0191] In addition, because the conversion efficiency of first
crystal 63 is relatively low, a large portion of the fundamental
waves (wavelength .lambda.) in incident light I0 pass though first
crystal 63 without being converted, and are included in radiated
light 11. However, because the conversion efficiency of second
crystal 64 is high, it is possible to obtain a high power second
harmonic wave (wavelength .lambda./2) as radiated light I2.
Further, the fundamental waves remaining in radiated light I2 are
separated at prism 65, so that only the second harmonic wave can be
output. Note that a separator may be employed in place of prism
65.
[0192] The present embodiment employs a first crystal 63 and a
second crystal 64 having a specific relationship with respect to
their resistance to light damage and their conversion efficiency.
As a result, the deficits of each crystal are compensated for, so
that an overall high conversion efficiency and high resistance to
light damage can be realized. Accordingly, the high power second
harmonic wave can be obtained with good efficiency.
[0193] Note that while the discussion of the preceding embodiments
concerned itself with the second harmonic wave, the present
invention can be broadly applied to sum frequency wave generation.
For example, when generating the third harmonic wave, the laser
oscillating system according to the embodiment shown in FIG. 8 can
be composed by replacing separators 61 and 62 with a mirror that
reflects both the fundamental and second harmonic waves, and by
eliminating beam damper 66. Alternatively, a design is also
possible in which any one of separators 61 and 62 and beam damper
66 are omitted, and fundamental and second harmonic waves from an
oscillator that simultaneously generates fundamental and second
harmonic waves are made to input to first crystal 63 directly.
EXAMPLES
Example 1
[0194] In the laser oscillating system shown in FIG. 8, LB4 which
was 5 mm.times.5 mm in cross section and had a length of 35 mm was
employed for first crystal 63, and BBO which was 5 mm.times.5 mm in
cross section and had a length of 7 mm was employed for second
crystal 64. Second harmonic waves were generated using green laser
with an average power of 30 W, a repetition frequency of 10 kHz, a
pulse width of 30 n sec, a beam diameter of 0.5 mm and a wavelength
of 532 nm, as incident light I0.
[0195] The average peak power density of incident light I0 was 51
MW/cm.sup.2 at this time, while the power and average peak power
density of the fundamental waves (532 nm) that remained in radiated
light I1 were 28.5 W and 48.4 MW/cm.sup.2, respectively.
Ultraviolet light (266 nm) with a stable output of 6.3 W was
obtained as radiated light I2.
[0196] Note that when a typical beam (532 nm) having an average
peak power density of 48.2 MW/cm.sup.2 is input to BBO, bulk damage
will occur. In the case of this example, however, the practical
peak power density is decreasing, as explained by FIG. 9, so that
bulk damage did not occur.
Comparative Example 1
[0197] BBO that was 5 mm.times.5 mm in cross section and had a
length of 7 mm was employed for both first crystal 63 and second
crystal 64 in the laser oscillating system shown in FIG. 8. The
conditions for the incident light I0 were the same as those
employed in Example 1.
[0198] In this case, however, the BBO incurred bulk damage from
incident light I0, so that it could not be used.
Example 2
[0199] In the laser oscillating system shown in FIG. 8, LB4 which
was 5 mm.times.5 mm in cross section and had a length of 35 mm was
employed for first crystal 63, and CLBO which was 5 mm.times.5 mm
in cross section and had a length of 10 mm was employed for second
crystal 64. Second harmonic waves were generated using a green
laser as incident light I0 under the same conditions as in Example
1, i.e., with an average power of 30 W, a repetition frequency of
10 kHz, a pulse width of 30 n sec, a beam diameter of 0.5 mm and a
wavelength of 532 nm.
[0200] The average peak power density of incident light I0 was 51
MW/cm.sup.2 at this time, while the power and the average peak
power density of the fundamental waves (532 nm) that remained in
radiated light 11 were 28.5 W and 48.4 MW/cm.sup.2, respectively.
Ultraviolet light (266 nm) with a stable output of 6.5 W was
obtained as radiated light I2.
[0201] Note that when a typical beam (532 nm) having an average
peak power density of 48.4 MW/cm.sup.2 is input to CLBO, bulk
damage will occur. In the case of this example, however, the
practical peak power density is decreasing, as explained by FIG. 9,
so that bulk damage did not occur.
Comparative Example 2
[0202] CLBO that was 5 mm.times.5 mm in cross section and had a
length of 10 mm was employed for both first crystal 63 and second
crystal 64 in the laser oscillating system shown in FIG. 8. The
conditions for the incident light I0 were the same as those
employed in Examples 1 and 2.
[0203] In this case, however, the CLBO incurred bulk damage from
incident light I0, so that it could not be used.
Comparative Example 3
[0204] In the laser oscillating system shown in FIG. 8, LB4 that
was 5 mm.times.5 mm in cross section and had a length of 35 mm was
employed for both first crystal 63 and second crystal 64. The
conditions for incident light I0 were the same as those employed in
Examples 1 and 2.
[0205] The average peak power density of incident light I0 was 51
MW/cm.sup.2 at this time, while the power and the average peak
power density of the fundamental waves (532 nm) that remained in
radiated light I1 were 28.5 W and 48.4 MW/cm.sup.2, respectively.
Ultraviolet light (266 nm) with an output of 3 W was obtained as
radiated light I2.
[0206] While the output of radiated light I2 was stable and bulk
damage did not occur, the radiated light output in this comparative
example was low as compared to the 6.3 W obtained in Example 1 and
the 6.5 W obtained in Example 2.
Example 3
[0207] A mirror for reflecting both fundamental and second harmonic
waves was employed in place of separators 61 and 62, and beam
damper 66 was omitted from the laser oscillating system according
to the embodiment shown in FIG. 8. Sum frequency waves were then
generated using this thus-modified laser oscillating system.
[0208] LB4 that was 5 mm.times.5 mm in cross section and had a
length of 35 mm was employed for first crystal 63, and LBO which
was 5 mm.times.5 mm in cross section and had a length of 15 mm was
employed for second crystal 64 in this laser oscillating system. A
fundamental wave laser having a wavelength of 1064 nm and an
average power of 10 W, and second harmonic waves having a
wavelength of 532 nm and an average power of 10 W, were employed
for incident light I0, to generate an ultraviolet laser of
wavelength 355 nm, which is the third harmonic wave.
[0209] Regarding this incident light I0, the repetition frequency
was 10 kHz, the fundamental wave laser's pulse width and beam
diameter were 30 n sec and 0.3 mm, respectively, and the second
harmonic wave laser's pulse width and beam diameter were 27 n sec
and 0.2 mm, respectively.
[0210] The average peak power densities of the fundamental wave
laser and the second harmonic wave laser in incident light I0 at
this time were, respectively, 47 MW/cm.sup.2 and 118 MW/cm.sup.2.
The power of the fundamental waves that remained in radiated light
I1 and the second harmonic waves were, respectively, 9.5 W and 9.5
W, and the average peak power densities of these fundamental waves
and second harmonic waves were, respectively, 45 MW/cm.sup.2 and
112 MW/cm.sup.2. Third harmonic waves (355 nm) having a stable
output of 5 W were obtained as radiated light I2.
[0211] Note that when a typical beam (532 nm) in which the average
peak power density is 112 MW/cm.sup.2 is input to LBO, bulk damage
will occur. In the case of this example, however, the practical
peak power density is decreasing, as explained by FIG. 9, so that
bulk damage did not occur.
Comparative Example 4
[0212] LBO that was 5 mm.times.5 mm in cross section and had a
length of 10 mm was employed for both first crystal 63 and second
crystal 64 in a laser oscillating system equivalent to that of
Example 3. The conditions for the incident light I0 were the same
as those employed in Example 3.
[0213] In this case, however, the LBO gradually incurred bulk
damage from the second harmonic waves (532 nm) in incident light
I0, so that the duration of time during which it could be employed
was limited, i.e., stable use over a long period of time was not
possible.
Comparative Example 5
[0214] LB4 that was 5 mm.times.5 mm in cross section and had a
length of 35 mm was employed for both first crystal 63 and second
crystal 64 in a laser oscillating system equivalent to that of
Example 3. The conditions for the incident light I0 were the same
as those employed in Example 3.
[0215] Third harmonic waves (355 nm) with a stable output of 2 W
were obtained as radiated light I2 in this case.
[0216] While the output of radiated light I2 was stable and bulk
damage did not occur, however, the output value was low when
compared to the 5 watts obtained in Example 3.
Example 4
[0217] A mirror for reflecting both first and second fundamental
waves was employed in place of separators 61 and 62, and beam
damper 66 was omitted from the laser oscillating system according
to the embodiment shown in FIG. 8. Sum frequency waves were then
generated using this thus-modified laser oscillating system.
[0218] LB4 which was 5 mm.times.5 mm in cross section and had a
length of 20 mm was employed for first crystal 63, and BBO which
was 5 mm.times.5 mm in cross section and had a length of 15 mm was
employed for second crystal 64 in this laser oscillating system.
Third harmonic waves of an Nd:YAG laser (first fundamental wave)
having a wavelength of 355 nm and an average power of 5 W, and a
Ti: sapphire laser (second fundamental wave) having a wavelength of
828 nm and an average power of 5 W, were employed as the incident
light I0, to generate ultraviolet laser of wavelength 248 nm, which
is the sum frequency wave.
[0219] Regarding this incident light I0, the repetition frequency
was 10 kHz, the first fundamental wave's (355 nm) pulse width and
beam diameter were 25 n sec and 0.2 mm, respectively, and the
second fundamental wave's (828 nm) pulse width and beam diameter
were 15 n sec and 0.2 mm, respectively.
[0220] The average peak power densities of the first fundamental
wave (355 nm) and the second fundamental wave (828 nm) in incident
light I0 were, respectively, 64 MW/cm.sup.2 and 106 MW/cm.sup.2.
The power of the first fundamental wave (355 nm) that remained in
radiated light I1 and the second fundamental wave (828 nm) were,
respectively, 4.85 W and 4.85 W. The average peak power densities
were 62 MW/cm.sup.2 and 103 MW/cm.sup.2, respectively. Sum
frequency waves (248 nm) with a stable output of 0.8 W were
obtained as radiated light I2.
[0221] Note that when a typical beam (355 nm) in which the average
peak power density is 62 MW/cm.sup.2 is input to BBO, bulk damage
will occur. In the case of this Example, however, the practical
peak power density is decreasing, as explained by FIG. 9, so that
bulk damage did not occur.
Comparative Example 5
[0222] BBO that was 5 mm.times.5 mm in cross section and had a
length of 15 mm was employed for both first crystal 63 and second
crystal 64 in a laser oscillating system equivalent to that of
Example 4. The conditions for the incident light I0 were the same
as those employed in Example 4.
[0223] In this case, however, the BBO gradually incurred bulk
damage from the first fundamental waves (355 nm) in incident light
I0, so that the duration of time during which it could be employed
was limited, i.e., stable use over a long period of time was not
possible.
Comparative Example 6
[0224] LB4 that was 5 mm.times.5 mm in cross section and had a
length of 20 mm was employed for both first crystal 63 and second
crystal 64 in a laser oscillating system equivalent to that of
Example 4. The conditions for the incident light I0 were the same
as those employed in Example 4.
[0225] Sum frequency waves (248 nm) with a stable output of 0.4 W
were obtained as radiated light I2 in this case.
[0226] While the output of radiated light I2 was stable and bulk
damage did not occur, the output value was low when compared to the
0.8 W obtained in Example 4.
INDUSTRIAL APPLICABILITY
[0227] As explained in detail above, in the wavelength converting
method and wavelength converting system according to the present
invention, wavelength conversion is carried out to incident light
that has a peak power density that is below, but near, the optimal
peak power density. As a result, a stable high conversion
efficiency can be achieved using a nonlinear optical crystal single
crystal, and in particular lithium tetraborate LB4. Thus, by means
of the present invention, an all solid state ultraviolet laser
oscillator, which offers durability with respect to practical
applications, can be employed.
[0228] In addition, the optimal peak power density can be easily
obtained using the program and recording medium according to the
present invention. Thus, an operator operating a wavelength
converting system in which radiated light of wavelength {fraction
(1/2)} .lambda. is obtained by causing light of wavelength .lambda.
to input to a nonlinear optical crystal under conditions of a
prescribed repetition frequency and crystal length, is able to set
a suitable peak power density.
[0229] In addition, because it is possible to increase the optimal
peak power density in the wavelength converting method and
wavelength converting system according to the present invention, a
stable output can be obtained even if the peak power density of the
incident light is high. As a result, stable high conversion
efficiency can be achieved using a nonlinear optical crystal such
as lithium tetraborate single crystal LB4. Accordingly, an all
solid state ultraviolet laser oscillator, which offers durability
with respect to practical applications, can be employed.
[0230] Moreover, by combining different types of nonlinear optical
crystals that have a specific relationship to one another in the
present invention, the individual deficits of the various crystals
are compensated for, so that a high conversion efficiency and high
resistance to light damage can be realized overall. Accordingly,
such high power sum frequencies as second harmonic waves can be
obtained with good efficiency.
* * * * *